1. Field of the Invention
This application relates generally to semiconductor processing equipment and specifically to an apparatus for delivering gases in an atomic layer deposition (ALD) device.
2. Description of the Related Art
There are several vapor deposition methods for depositing thin films on the surface of substrates. These methods include vacuum evaporation deposition, Molecular Beam Epitaxy (MBE), different variants of Chemical Vapor Deposition (CVD) (including low-pressure and organometallic CVD and plasma-enhanced CVD), and Atomic Layer Epitaxy (ALE), which is more recently referred to as Atomic Layer Deposition (ALD).
In an ALD process, one or more substrates with at least one surface to be coated are introduced into the reactor or deposition chamber. The substrate is heated to a desired temperature above the condensation temperature of the selected vapor phase reactants. One reactant is capable of reacting with the adsorbed species of a prior reactant to form a desired product on the substrate surface. The product can be in the form of a film, liner or layer on the substrate.
ALD is a well known process in the semiconductor industry for forming thin films of materials on substrates such as silicon wafers. ALD is a type of vapor deposition wherein a film is built up through deposition of multiple ultra-thin layers with the thickness of the film being determined by the number of layers deposited. In an ALD process, gaseous molecules of one or more compounds (precursors) of the material to be deposited are supplied to the substrate or wafer to form a thin film of that material on the wafer. In one pulse, a first precursor material is adsorbed largely intact in a self-limiting process on the wafer. The process is self-limiting because the vapor phase precursor cannot react with or adsorb upon the adsorbed portion of the precursor. After any remaining first precursor material is removed from the wafer or chamber, the adsorbed precursor material on the substrate may be decomposed or reacted within a subsequent reactant pulse to form no more than a single molecular layer of the desired material. The subsequent reactant may, e.g., strip ligands from the adsorbed precursor material to make the surface reactive again, replace ligands and leave additional material for a compound, etc. In practice, less than a monolayer is formed per cycle on average due to steric hindrance, whereby the size of the precursor molecules prevent access to adsorption sites on the substrate, which may become uncovered through subsequent cycles. Thicker films are produced through repeated growth cycles until the target thickness is achieved.
Reactants and temperatures are typically selected to avoid both condensation and thermal decomposition of the reactants during the process, such that chemical reaction is responsible for growth through multiple cycles. However, in certain variations on ALD processing, conditions can selected to vary growth rates per cycle, possibly beyond one molecular monolayer per cycle, by hybridizing CVD and ALD reaction mechanisms. Other variations maybe allow some amount of spatial and/or temporal overlap between the reactants. In ALD and variations thereof, two, three, four or more reactants can be supplied in sequence in a single cycle, and the content of each cycle can be varied to tailor composition.
During a typical ALD process, the reactant pulses, all of which are typically in vapor or gaseous form, are pulsed sequentially into a reaction space (e.g., reaction chamber) with removal steps between reactant pulses to avoid direct interaction between reactants in the vapor phase. For example, inert gas pulses or “purge” pulses can be provided between the pulses of reactants. The inert gas purges the chamber of one reactant pulse before the next reactant pulse to avoid gas phase mixing. A characteristic feature of ALD is that each reactant is delivered to the substrate until a saturated surface condition is reached. The cycles are repeated to form an atomic layer of the desired thickness. To obtain a self-limiting growth, a sufficient amount of each precursor is provided to saturate the substrate. As the growth rate in each cycle of a true ALD process is self-limiting, the rate of growth is proportional to the repetition rate of the reaction sequences rather than to the flux of reactant.